German Patent Document DE-OS 39 26 821 describes a container in which the upper wall or cover of the container body is provided with filling openings at least in part by pressing, drawing or otherwise deforming sheet metal of the cover to form a tubular fitting surrounding the filling opening so that the tubular fitting is composed of thin wall sheet metal and is provided in one piece with the cover.
In this system, an internal screw thread can be pressed into the tubular setting or machined therein, The screw thread is adapted to engage in an externally threaded stopper or plug which is screwed into the tubular fitting.
A seal is formed by the plug or stopper against the cover of the container or against a shoulder formed by the tubular fitting itself and, for this sealing action the plug can thereby be seated against the cover or the tubular fitting and sealing between the tubular fitting and the cover does not pose a problem since the fitting is formed in one piece with the cover portion of the container.
This type of system, however, presents a significant problem when the material from which the tubular fitting is formed and is thus drawn from the cover, is relatively thin. The formation of screw threads therein becomes difficult and the contours of the internal screw thread may be lost. The problem can be only partially solved by coining the material of the tubular fitting to make it thicker.
By and large, therefore, it is difficult to provide sufficiently deep screw threads as usually are necessary for effective retention of the plug or stopper in such thin materials. Indeed, it has been proposed to form the tubular fittings from relatively thick materials and to weld them to the top wall or cover of the container. The threaded bushings used for this purpose require an additional and expensive welding operation and frequently present a problem with respect to effectiveness of the weld and thus the seal between the threaded bushing and the cover.
In German Patent Document 3,934,210, an internal screw thread for a tubular fitting is formed by a helical spring composed of steel wire and received in the tubular fitting and welded thereto. This system requires expensive welding operations or deformation steps to anchor the helical spring with its turns in the proper spacing or against the axial stresses which are applied thereto when the threaded stopper or plug is tightened in the tubular fitting.
The U.K. Patent Document GB 741,559 describes a beer barrel fabricated from aluminum and having filling openings in which bungs are welded. To enable problem-free welding, the bung is also composed of a light metal such as aluminum. However, it has been found that a bung of this type is excessively soft so that repeated closure by plugs or stoppers or the like or the connection of pipes to the bung will give rise to substantial wear. This is a significant drawback with respect to the reuse of the barrel.
In the relatively thick-wall bung, a screw thread can be formed in which a threaded bushing of harder material, preferably steel, can be screwed. This bushing is formed with a second screw thread, for example, a trapezoidal-profile internal thread, for engagement with the stopper or the like. This, of course, allows the internal screw thread of the bushing to be relatively shallow, but it does require a thick wall construction with a high consumption of relatively expensive material and frequently also requires an expensive welding process if an absolute seal is to be provided between the bushing and the bung.